1. Field of the Invention
The present invention relates to an isolation transformer arrangement and in particular to an arrangement suitable for providing an isolation barrier in medical equipment.
2. Description of the Prior Art
Many types of medical equipment include sensors which are in contact with a patient. Although these sensors operate at low voltage and current levels that do not present a shock hazard to the patient that hazard can occur if an electrical short circuit occurs within the equipment or if other equipment connected to the patient develops a fault and the relatively high voltage and current levels from an external power supply line are supplied to the sensors contacting the patient.
For these reason regulatory authorities of many countries, such as for example the F.D.A. in the USA which requires compliance with IEC 60606-1, specify that medical equipment must be designed with an isolation barrier between circuits containing patient connections and circuits connected to power supply line voltages. Such isolation barriers must isolate, against several kilovolts AC with a leakage current of only several microamperes when the supply line voltage is applied across the isolation barrier. Typically, suitable isolation barriers are formed using isolation transformers, usually mounted on a printed circuit board (PCB) containing the circuits to be isolated. Generally, one of the two circuits between which an isolation barrier is required is electrically connected to the primary transformer winding or windings and the other circuit to the secondary transformer winding or windings.
One known type of isolation transformer is a xe2x80x9cbobbin-typexe2x80x9d isolation transformer which has a hollow plastic former or bobbin about which wires are wound to form the primary and the secondary windings and through which a ferrite core piece passes. The primary and secondary windings may be wound concentrically, one on top of the other, or may be displaced from one another along the core to increase the so called xe2x80x9ccreepage distancexe2x80x9d. The necessary isolation may be achieved by sheathing the wire of the two sets of windings in a suitable insulating material. This provides a transformer which is relatively inexpensive to produce and one in which faulty windings can be readily replaced.
However, the isolation achieved in this type of transformer is largely due to the separation and insulation between wires and the windings which mitigates against reducing the size of the transformer for mounting on the PCB. Size of the transformer can become a major issue since the medical device typically needs to accommodate several PCBs, one or some of which may have mounted thereon isolation transformers, in as small a volume as possible. Moreover, complicated tapping arrangements for the windings are difficult to achieve in wire wound transformers and often lead to a high failure rate and a consequent increased unit cost.
Low-profile planar transformers are also well known as isolation barriers. In such transformers the primary and secondary windings are each formed by electrically conducting runs, usually on an insulating planar surface such as a surface of a PCB, for example a multi-layer PCB, and arranged so that successive runs are separated by an insulating PCB layer to provide at least part of the necessary isolation. The layer or layers that constitute each of the windings are then usually magnetically coupled by means of an inductive core member. Forming the windings on a PCB also provides an increased ease of tapping selected conducting traces to provide a selectable transformer output voltage as compared to tapping selected windings of a wire wound transformer. This also allows complex tapping arrangements to be constructed relatively simply and consistently.
However the bonding of the PCB layers is usually done by gluing which also contributes to the isolation but can lead to uncontrolled variations in the dielectric properties of the inter-trace insulation, for example through the uncontrolled formation of air bubbles within the glue as it is applied. This is of particular concern for the insulation between the primary and the secondary windings as it may adversely effect the isolation provided by the transformer This leads to the necessity for increased quality control and hence higher unit costs.
It is also known from PCT Application WO 99/31683 to provide a xe2x80x9chybridxe2x80x9d low profile transformer power supply formed by a flat winding primary coil magnetically coupled to a secondary coil having a winding pattern deposited on a substrate such as a PCB. The transformer is designed specifically for mounting outside a periphery of a PCB which carries circuitry to be powered from the transformer.
It is an object of the present invention to provide an isolation transformer of relatively small size in which the isolation characteristics can be readily controllable and which can be integrated with a PCB carrying circuitry to be isolated.
The above object is achieved in accordance with the principles of the present invention in an isolation transformer arrangement having an isolation transformer with magnetically coupled primary and secondary windings, one of which is formed by at least one insulating substrate with a planar conductor run on a surface thereof, and the other of which is formed by a number of turns of an insulated wire conductor, and wherein the insulating substrate is a multi-layer printed circuit board also carrying two electrically separate circuits respectively connectable to the primary and secondary windings, and wherein the insulation of the wire conductor provides a desired level of electrical isolation between the two circuits.
By providing one winding as a planar conductive trace on the PCB board carrying circuitry to be isolated by the transformer a reduction in size and an ease of tapping as compared with an all wire transformer is achieved and by providing an insulated wire winding substantially all of the electrical isolation necessary for medical use can be achieved by a suitable tailoring of that insulation in a manner well known in the art. Moreover, the isolation can be tested before the wire is turned to provide the transformer winding, thereby reducing the possibility of the completed transformer being rejected during quality control.
Usefully the wire may be turned about a hollow bobbin similar to the known bobbin type transformer arrangement or other former, such as a leg of an E-core ferrite element, to provide for ease of collocation of the primary and secondary windings into the final transformer. The bobbin (or former) and the planar windings may be releasably replaceable which has the advantage that, since the isolation is provided by the insulated wire, winding poor isolation caused by faulty insulation in an assembled transformer can be easily remedied without replacing the entire transformer and hence the entire circuitry contained on the PCB board.